The present invention relates to a cell constituted by the housing of an electricity generating element such as a lithium ion secondary cell in an aluminum casing, and, more particularly, to a cell in which electrical connection with the aluminum casing, which is an electrode part, is easily arranged.
By way of example, secondary cells used as power sources for portable electronic instruments are required to be of a high energy density and, at the same time, are required to be of a shape efficient in terms of space used in order to achieve weight-lightness and compactness. A lithium ion secondary cell which uses a square aluminum casing is attracting the spotlight as a cell which fulfills these requirements.
Because the lithium ion secondary cell, in terms of its structure apart from anything else, is required to be hermetically stable over a long time, an aperture-sealing plate is bonded by laser welding to the open end of a square casing with a bottom, thereby sealing the open end. A rivet which serves as a negative electrode terminal is attached to the sealing plate such as to be insulated therefrom, and the square casing is used as a positive electrode terminal, thereby constituting both electrode terminals of the cell.
To both of the positive and negative terminals, it is necessary to connect a lead in order to execute electrical connection with the apparatus which uses the cell. In the case where the square casing is formed from an aluminum material, resistance welding and soldering are difficult, wherefore a construction is adopted in which a metal plate, which allows easy lead connection, is bonded to the square casing. Such a metal plate may be obtained by the cladding bonding of an aluminum plate and a nickel plate, and by ultrasonically welding it to the bottom surface of the square casing, resistance welding or soldering of the lead to the nickel plate is made simple. Usage of such lead is taught, for example, in Japanese Laid-open Patent Application 9-320565. However, there is a problem that the bond between the lead plate and the casing is not rigid enough unless the layer of the aluminum in the cladding material is thick enough relative to the nickel plate.
Many means of resistance welding a lead to the nickel plate of the abovementioned cladding material are used for connecting the lead to the square casing. The welding operation must be performed quickly so as not to cause a thermal effect inside the square casing. For that reason, a material of a type which has better resistance welding properties than the nickel plate, or a material of a type which is not liable to have a thermal effect is desired.